A detailed study is proposed of the equilibria involved in the formation of cytoplasmic microtubules, and of the kinetics of their assembly from and disassembly to their constituent protein subunits. Four complimentary techniques of biophysical chemistry will be employed to attack the problem: low angle light scattering, turbidity measurement, velocity and equilibrium ultracentrifugation, and electron microscopy. The work is aimed first at characterizing thoroughly the formation of microtubules from pure tubulin. Conditions will be found under which microtubules are in reversible equilibrium with their subunits. Equilibrium constants as well as rate constants of the assembly and disassembly processes will be measured, and attention will be given to the testing of realistic models of the processes. Second, as time allows, the effects of selected variables (non-tubulin proteins, Ca ions, GTP, etc.) thought to be modulators of assembly and disassembly in vivo will be explored and interpreted against the background of information gained with pure tubulin alone. Particular questions to be asked about these presumed modulators of assembly and disassembly are whether they exert their effects through alteration of the kinetics or the equilibria of microtubule formation, and which particular steps (nucleation, growth, disassembly, dissolution of preformed nuclei) each one alters. The study is to be carried out by existing methods, for the most part. Its novel aspects include a proposal to use glutaraldehyde-fixed and fractionated microtubules to cross-calibrate the techniques experimentally, a primary emphasis on finding conditions of reversible association, and an equal emphasis on studying disassembly kinetics with assembly kinetics.